High brightness solid-state laser with zig-zag amplifier

ABSTRACT

A solid-state laser architecture producing a beam of extremely high quality and brightness, including a master oscillator operating in conjunction with a zig-zag amplifier, an image relaying telescope and a phase conjugation cell. One embodiment of the laser architecture compensates for birefringence that is thermally induced in the amplifier, but injects linearly polarized light into the phase conjugation cell. Another embodiment injects circularly polarized light into the phase conjugation cell and includes optical components that eliminate birefringence effects arising in a first pass through the amplifier. Optional features permit the use of a frequency doubler assembly to provide output at twice optical frequencies, and an electro-optical switch or Faraday rotator to effect polarization angle rotation if the amplifier material can only be operated at one polarization. The zig-zag amplifier is cooled by flow of cooling liquid, preferably using longitudinal flow to minimize temperature gradients in a vertical direction, and has cooling channel seals disposed in dead zones that receive no light, to minimize optical damage to the seals. Light is input to the amplifier at a near normal angle of incidence, to minimize polarization by reflection and to permit a polarizer to be used to extract an output beam from the amplifier. Antireflective coatings on edges and on sides of the amplifier eliminate parasitic oscillations, and wedge-shaped windows provide uniform pumping by eliminating gaps between diode arrays.

BACKGROUND OF THE INVENTION

This invention relates generally to high-power solid-state lasers and,more particularly, to systems for producing laser beams of extremelyhigh brightness. Solid-state lasers with an average power up to 100 W(watts), and even higher powers, are needed in a variety of military,industrial and commercial applications, including X-rayphotolithography, laser machining and drilling, space and underwatercommunication, and medical applications.

The brightness of a laser beam is proportional to the average power andis inversely proportional to the square of the beam quality, where thebeam quality is in turn defined in relation to a diffraction-limitedbeam, i.e., a diffraction-limited beam has an ideal beam quality of 1.0.A worse beam quality of, say, 1.5 results in a brightness of 1/(1.5)² or44.4% of the brightness of the diffraction limited beam. Since thebrightness falls off in proportion to the square of the beam quality, itis extremely important to control the beam quality if high brightness isa design goal.

A number of laser architectures disclosed in various prior patents use aphase conjugated master oscillator power amplifier (PC MOPA)configuration, but still fail to produce a desirably bright beam, orhave other drawbacks. Accordingly, there is still a significant need foran improved solid-state laser architecture providing excellent beamquality and, therefore, an extremely bright beam. The present inventionsatisfies this need, as outlined in the following summary.

SUMMARY OF THE INVENTION

The present invention resides in a high brightness solid-state lasersource, comprising a master oscillator, generating a pulsed beam havinga nearly diffraction limited beam quality; a zig-zag slab amplifierpositioned to receive and amplify the beam from the master oscillator,during a first pass through the amplifier; a phase conjugation cellpositioned to receive the amplified beam from the zig-zag slab amplifierand to reflect the beam in phase conjugated form back into the zig-zagslab amplifier for a second pass, whereby aberrations introduced in thezig-zag slab amplifier during the first pass are practically canceledduring the second pass; and means for extracting an amplified beam afterthe second pass through the amplifier, whereby the extracted amplifiedbeam has high beam quality and extremely high brightness. The beamproduced has both high beam quality and high brightness, with averagebeam powers up to 100 watts or more.

In one preferred embodiment of the invention, the means for extractingan amplified beam includes a quarter-wave plate positioned between thezig-zag amplifier and the phase conjugation cell, to effect a rotationin polarization angle in the light beam input to the amplifier in thesecond pass; and a polarizer positioned between the master oscillatorand the zig-zag amplifier. The polarizer transmits light from the masteroscillator to the amplifier, and outcouples the orthogonally polarizedreturn beam from the amplifier.

Alternatively, the means for extracting an amplified beam includes apolarizer positioned at the output of the master oscillator; and meanspositioned between the polarizer and the amplifier, for rotating thepolarization angle of the return beam from the amplifier. As in thefirst-mentioned embodiment, the polarizer transmits light from themaster oscillator to the amplifier and outcouples the differentlypolarized return beam from the amplifier.

In another variant of the invention, the means for extracting anamplified beam includes a frequency doubler assembly positioned betweenthe master oscillator and the zigozag amplifier, to effect outcouplingof optical light from the source.

In yet another embodiment of the invention, a quarter-wave plate is notused in conjunction with the phase conjugation cell. Instead, a Faradayrotator positioned adjacent to the phase conjugation cell rotates thepolarization angle of both the input beam and the return beam by 45°, toproduce an orthogonally polarized output beam that can be extractedusing a polarizer. The Faraday rotator also serves to facilitate removalof birefringence effects arising in the amplifier. The only drawback ofthis embodiment is that linearly polarized light is injected into thephase conjugation cell, which may not then operate at desirably highenergy levels.

Yet another embodiment of the invention also uses a quarter-wave plate,to provide circularly polarized light to the phase conjugation cell, butalso includes means for eliminating birefringence resulting from thefirst pass through the amplifier. The means for eliminating first-passbirefringence includes first polarization-sensitive means locatedbetween the amplifier and the quarter-wave plate, for transmitting aprincipal component of the amplified input beam along a first opticalpath, and simultaneously reflecting an orthogonal birefringencecomponent of the amplified input beam along a second optical path;optical means located in the first optical path, for rotating thepolarization angle of the principal component by 90°; means located inthe second optical path, for transmitting light in the forward directionwithout change in the polarization angle, and for rotating thepolarization angle 90° for light traversing the second optical path inthe return direction, whereby the birefringence component of the inputbeam traverses the second optical path unaffected; and secondpolarization-sensitive means, located at a junction of the first andsecond optical paths, wherein the principal component of the input beamis reflected from the first optical path into the quarter-wave plate,the birefringence component is reflected into space and discarded, andthe return beam from the quarter-wave plate is transmitted back alongthe second optical path. The return beam is subjected to a 90° rotationin polarization angle by the optical means in the second optical path,and is reflected by the first polarization-sensitive means back towardthe amplifier.

More specifically, the first and second polarization-sensitive means arepolarizers that reflect light that is linearly polarized in onedirection and transmit light that is linearly polarized in an orthogonaldirection. In the illustrative embodiment, each of the first and secondoptical paths also includes a mirror.

The invention may also be defined in terms of a method for eliminatingbirefringence effects introduced in a first pass of an input light beamthrough an optical amplifier. The method comprises the steps ofseparating the amplified input beam into a principal component and abirefringence component having first and second orthogonal linearpolarization angles; rotating the polarization direction of theprincipal component by 90°, such that it also has the secondpolarization angle; directing the birefringence component and thepolarization-rotated principal component onto a polarization-sensitivereflector, wherein the birefringence component is reflected into spaceand discarded and the principal component is reflected into aquarter-wave plate; converting the linearly polarized principalcomponent into circularly polarized light; reflecting the circularlypolarized light from a phase conjugation cell and back through thequarter-wave plate; converting the circularly polarized reflected beaminto linearly polarized light having the first polarization angle;transmitting the return beam through the polarization-sensitivereflector and along the second optical path; rotating the polarizationangle of the return beam using an optical component in the secondoptical path that had no effect on the input beam; reflecting the returnbeam back into the amplifier, without any birefringence component andwith the second polarization angle; and extracting the return beam as anoutput beam, using another polarization-sensitive reflector.

The present invention also resides in a zigozag slab amplifier includinga slab of solid-state laser material, the slab having two opposingsidewalls from which a light beam is repeatedly reflected as itprogresses longitudinally through the amplifier, and two opposingvertical end walls, oriented to allow input of a light beam at an anglepractically normal to the end walls, whereby input of light at a nearnormal angle of incidence provides low-loss injection independent ofbeam polarization. The amplifier also has at least one diode arraypositioned in close proximity to at least one of the sidewalls, toprovide power to the amplifier; and means for cooling the sidewalls byflowing a liquid in contact with the sidewalls.

The means for cooling the sidewalls includes means for flowing theliquid in a longitudinal direction past the sidewalls, to minimizetemperature gradients in a vertical direction. The means for flowing theliquid in a longitudinal direction includes a transparent windowpositioned adjacent to and parallel to each sidewall, forming a channelfor the flow of cooling liquid; and a seal between the transparentwindow and the sidewall, the seal having transverse segments positionedon the sidewall in dead zones on which no light falls. The angle ofincidence of light internally on each sidewall is selected in relationto the beam width to form the dead zones. Preferably, the transparentwindows are wedge-shaped, to compensate for gaps between arrays ofdiodes and provide a more uniform pumping action in the verticalvertical direction.

The slab further includes antireflective coatings on its end walls andon sidewall portions on which the input light beam first impinges,whereby parasitic light rays approximately parallel with thelongitudinal direction are suppressed by the antireflective coatings. Inaccordance with another feature of the invention, the zig-zag amplifierfurther comprises thermal control means positioned at upper and loweredge faces of the amplifier slab, to improve temperature gradients inthe vertical direction, and antireflective coatings on the upper andlower edge faces, to suppress parasitic light rays having a verticalcomponent.

It will be appreciated from the foregoing that the present inventionrepresents a significant advance in the field of high brightness lasersources of moderate to high power. Additional aspects and advantages ofthe invention will become apparent from the following more detaileddescription, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical schematic diagram of a phase conjugated masteroscillator power amplifier (PC MOPA) configuration embodying theprinciples of the present invention;

FIG. 2a is an end view of a master oscillator rod assembly used in theconfiguration of FIG. 1;

FIG. 2b is a top view of the master oscillator rod assembly of FIG. 2a;

FIG. 3 is an optical schematic diagram of a master oscillator resonatorused in the configuration of FIG. 1;

FIG. 4 is an end view of a laser head (amplifier) in accordance with theinvention;

FIG. 4a is a perspective view of a diode array module used in the laserhead of FIG. 4;

FIG. 5 is a fragmentary top view of a zig-zag amplifier used in theconfiguration of FIG. 1;

FIG. 6 is a schematic diagram illustrating operation of a phaseconjugation cell;

FIGS. 7a, 7b and 7c are optical schematic diagrams showing threealternative techniques for extracting energy from the laserconfiguration of the invention;

FIG. 8 is an optical schematic showing a preferred configuration forbeam shaping in the master oscillator of the invention;

FIG. 9 is an alternative embodiment of the invention using imagerelaying telescopes and multiple amplifiers; and

FIG. 10 is a graph showing the variation of extracted energy and beamquality, respectively, with master oscillator energy.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in the drawings for purposes of illustration, the presentinvention pertains to a solid-state laser architecture for generating alaser beam of moderate to high power and extremely high brightness, andto a related laser amplifier structure. Although the phase-conjugatedmaster oscillator power amplifier (PC MOPA) configuration has been usedin many variations, all have to date suffered from less than outstandingbeam quality and, therefore, diminished brightness levels.

In accordance with one aspect of the present invention, a MOPAconfiguration uses a zig-zig amplifier and provides a beam of extremelyhigh quality, approximately 1.1 times the diffraction limit, and acorrespondingly high brightness. The brightness is defined as: ##EQU1##where: P_(ave) =average beam power,

λ=wavelength, and

BQ=beam quality.

The overall architecture is shown in FIG. 1 as including a masteroscillator, indicated by reference numeral 10, a beam shaping telescope12, a polarizer 14, a zig-zag amplifier 16, an image relaying telescope17, a quarter-wave plate 18, and a phase conjugation mirror 20. Theimage relaying telescope 17 includes a pair of lenses in an afocalconfiguration (as better shown at 112 in the embodiment of FIG. 9), andfunctions to eliminate the effects of diffraction from hard apertures,such as the amplifier 16. The image relaying telescope 17 also maintainsbeam uniformity. The master oscillator 10, which operates in conjunctionwith a Faraday isolator 24, produces a high quality, low energy opticalbeam of insufficient power for many purposes. The master oscillator beamis first passed through the beam shaping telescope 12, to furthercondition the beam, and enters the zig-zag amplifier 16. The amplifier16, which may consist of a string of amplifiers, amplifies the beam onits first pass and the beam then is image relayed onto the phaseconjugation mirror 20. Almost certainly, the amplifier causes phaseaberrations in optical wavefronts of the beam as it passes through theamplifier medium. However, as is well known, phase conjugation may beused to cancel these aberrations by passing a phase-conjugated form ofthe beam back through the amplifier.

Extraction of an output beam from the architecture shown in FIG. 1 iseffected by means of the quarter-wave plate 18 and the polarizer 14. Thequarter-wave plate rotates the polarization angle of the beam by meansof two passes through the plate. More specifically, on the first passthrough the quarter-wave plate 18 the linear polarization of the beam isconverted to circular polarization. On the return pass, the circularlypolarized beam is converted back to linearly polarized light, but with apolarization direction orthogonal to that of the original beam. Sincethe amplified beam leaving the amplifier 16 on the return pass has anorthogonal polarization with respect to the beam that was input to theamplifier, the polarizer 14 can be used to extract the output beam. Analternative extraction scheme uses an optional frequency doublingassembly 26, which extracts green light in the visible portion of thespectrum.

Another important aspect of the invention is the zig-zag amplifier 16.Although amplifiers of this type have been known for some years theyhave inherently not been able to provide exceptionally good beamquality. One reason for this is the temperature gradients that developduring operation of the amplifier. As shown in FIGS. 4, 4a and 5, azig-zag amplifier consists of a slab of a material, such asyttrium-aluminum-garnet crystal (YAG), through which a light beam ispassed, by repeated reflections from opposite sidewalls of the slab. Aportion of the slab is indicated by reference numeral 30. Inconventional zig-zag amplifiers, an input beam impinges on an edge faceof the slab at an angle selected to permit the beam to refract towardits first reflection point on a sidewall. Arrays of diodes, as indicatedat 32, are disposed on opposite sides of the slab 30 and provide theenergy for the amplification process. This optical pumping process alsocauses uneven heating in the slab 30. Cooling is effected by means ofwater channels 34 immediately adjacent to the sidewalls of the slab, andformed between the sidewalls and wedge-shaped windows 36 of sapphire orsimilar material. Prior to the present invention, cooling water waspumped in a direction perpendicular to the direction of progression ofthe beam through the amplifier, i.e. along the shorter or heightdimension of the sidewalls.

In accordance with one aspect of the present invention, cooling water isflowed longitudinally, i.e. along the longer dimension of the slab 30,as indicated by the arrows in FIG. 5, to minimize temperature gradientsalong the height dimension. The zig-zag path of the beam, as itprogresses along the longitudinal direction, tends to average outtemperature effects in the transverse direction.

In accordance with another important aspect of the invention, the beamis launched into the amplifier at a near normal (i.e. perpendicular)angle of incidence. This minimizes polarization caused by externalreflection from the slab edge and therefore permits the use of thequarter-wave plate 18 and the polarizer 14 to extract the amplified beamfrom the apparatus. As best shown in FIG. 5, an edge face 40 of the slab30 is formed at non-perpendicular angle to the sidewalls of the slab.The acute angle between the edge face 40 and the sidewall is selected sothat the angle the beam makes with the normal to the sidewall is greaterthan the critical angle of incidence for internal reflection from thesidewalls. Therefore, the light beam will be launched into the amplifierslab 30 with little or no refraction, and will be first reflected by asidewall, as indicated by the beam paths 42 and 44. The paths 42 and 44,which represent the outside edges of the beam, progress along the slab30 by making multiple reflections from the sidewalls. It will beobserved from FIG. 5 that the initial angle of incidence on a sidewallis chosen in relation to the slab thickness such that there will beshadow stripes formed at regular intervals along each sidewall. Forexample, the shadow stripes 46 and 48 are dead zones on which no portionof the beam impinges. The return path through the amplifier exactlytraces the beam path during its first pass, so the dead zones arepreserved after the second pass.

In accordance with another important aspect of the invention, selectedshadowed dead zones are used as the locations of segments of a pair ofwater seals, indicated at 50 and 52. Each seal is an O-ring or similarconfiguration, and is positioned between the slab 30 and thewedge-shaped windows 36. Each of the seals 50 and 52 follows anapproximately rectangular path, having longitudinal segments, shown inFIG. 4, and transverse segments, shown in FIG. 5, with appropriateopenings (not shown) to provide for flow of water into and out of thechannels 34. The importance of the location of the transverse segmentshas to do with the durability of the seals when exposed to opticaldamage by intense light. The longitudinal segments can be convenientlylocated above and below the active region of the amplifier, but thelocation of the transverse segments presents a problem since the beamimpinges repeatedly on the sidewalls of the slab 30. However, theinvention provides a solution to the problem by locating the transversesegments of the seals 50 and 52 in the deliberately shadowed dead zones,such as 46 and 48. Therefore, the entire length of each of the seals 50and 52 is located in a region that is permanently shaded from the lightbeam, and optical damage to the interface between the slab 30 and theseals 50 and 52 is virtually eliminated.

Another important aspect of the invention resides in the use of thewedge-shaped windows 36 between the diode arrays 32 and the amplifierslab 30. In addition to functioning as transparent outer walls of thewater channels 34, the windows also provide optical compensation for thegaps between the diode arrays 32, to give a more uniform pumping actionin the vertical direction.

One of the difficulties of operation of zig-zag amplifiers arises fromthe presence of "parasitics," in the form of light that takes anunintended path and interferes with the operation of the amplifier. Forexample, parasitic rays that are approximately longitudinal may bereflected from an end face of the slab 30 and impinge normally on asidewall of the slab. This could give rise to a lasing effect in theamplifiers, with light being repeatedly reflected back and forthperpendicularly between the sidewalls. To minimize this type ofparasitic effect, the amplifier of the invention includes anantireflective coating on the end faces of the amplifier slab 30 and ona region of the sidewall where the beam first impinges, between the endwall and the seal 52.

Another type of parasitic problem arises from rays that make repeatedreflections between opposite upper and lower edge faces of the slab 30.As shown in FIG. 4, parasitics of this type are suppressed by anotherantireflective coating 60 applied to the upper and lower edge faces. Thecoating 60 may be copper oxide or black paint.

The amplifier structure also includes means for controlling temperatureat the upper and lower edge faces of the slab 30. Each of the upper andlower edge faces of the slab 30 has a temperature control slot (62, 64)extending vertically into the slab and extending horizontally along itslength. Temperature in the slots 62, 64 is controlled either by the useof a liquid flowing in the slots, or by use of a heater bar embedded ina conductive medium in or near the slots. One of these mechanisms isused to control the temperature profile in a vertical direction, toallow the profile in the umpumped region of the slab to approximate thatin the pumped region.

The phase conjugation cell 20 is a conventional stimulated Brillouinscattering (SBS) cell, using a suitable SBS medium, such as liquid freon113 or gaseous nitrogen. As is well known, the SBS process reverses thewavefront of an input beam. (Portions of the wavefront that were laggingbecome leading, and vice versa.) Aberrations impressed on the wavefrontduring the first pass through the amplifier 16 are, therefore, negatedand virtually removed during the second pass after reflection from thephase conjugation cell 20. Of course, this technique eliminates theaberrations only if there have been no significant changes in theaberrating medium, i.e. the amplifier slab 30, between the first andsecond passes. Aberrations introduced in the amplifier are causedprincipally by temperature gradients, which change relatively slowly andare, therefore, effectively eliminated by the phase conjugation scheme.

The phase conjugation principle is illustrated diagrammatically in FIG.6. The beam generated by the master oscillator 10 and transmitted by thepolarizer 14 is shown as having an ideal wavefront, which is aberratedby the amplifier 16. After the first pass through the amplifier 16, thebeam passes through the quarter-wave plate 18 and is focused into theSBS cell 20 by a suitable lens 70. After reflection and phaseconjugation, the beam is still aberrated, but in an opposite phasesense, so the aberrations are canceled in the amplifier 16, and theaberration-free beam is extracted from the apparatus by means of thepolarizer 14.

FIGS. 7a, 7b and 7c show alternative techniques for extracting an outputbeam from the apparatus of the invention. As already discussed, thepreferred approach is to use the quarter-wave plate 18 in conjunctionwith the polarizer 14. However, if this approach cannot be used, perhapsbecause the amplifier uses a birefringent crystal that will operateeffectively in only one polarization mode, then an alternativeextraction technique can be used. Instead of the quarter-wave plate 18,the apparatus includes an electro-optical (EO) switch or Faraday rotator71, positioned immediately downstream of the polarizer 14. Basically,the switch or rotator 71 rotates the polarization angle of only thereturn beam, i.e. the beam emerging from the amplifier on the secondpass. If an EO switch is used, its operation is timed such that theswitch is activated only when a return pulse of the beam is expected.During each pulse of the input beam, the switch is inactive and does notaffect the beam, but the return pulse has its polarization angle rotatedto be orthogonal to that of the input beam. A Faraday rotator used forthe optical element 71 effects a 45° rotation of the polarization angleof both the input beam and the return beam. Therefore, the return beamhas an orthogonal polarization with respect to the input beam. and thepolarizer operates as before to extract the output beam.

In circumstances where there is significant thermally inducedbirefringence in the gain medium, it is desirable to eliminate or reducethe birefringence, since birefringence results in loss upon outcouplingand can damage the master oscillator 10 and the beam shaping optics 12.By replacing the quarter wave plate 18 with a Faraday rotator 71' (FIG.7b) the birefringence that is accumulated on the first pass through thegain medium is corrected on the return pass, restoring the polarizationpurity of the beam. The Faraday rotator 71' rotates the polarizationangle a total of 90°, which not only cancels the effects of anybirefringence in the amplifier 16, but also permits outcoupling throughthe polarizer 14.

Under the FIG. 7b configuration, the light injected into the SBS cell 20is mostly linearly polarized light. Some SBS media, such as liquids,have a much lower optical breakdown threshold for linearly polarizedlight than for circularly polarized light, which is the case when aquarter wave plate is used. A low breakdown threshold limits the rangeof energies over which the SBS cell 20 can be used, so that the goal ofhigh output energy and brightness may not be attained.

In cases where high energies are required, it would be preferable to usecircularly polarized light in the SBS cell 20, as provided by thequarter-wave plate 18 in the FIG. 1 configuration, but to provide inaddition a technique for substantially reducing the effects of thermallyinduced birefringence in the amplifier gain medium. For this purpose, analternative extraction method can be used to reduce the birefringence bya factor of two, by restoring the polarization purity of the beam at theSBS cell 20.

FIG. 7c shows an optical configuration that removes orthogonallypolarized light arising from birefringence, using a combination of twopolarizers 72, 73, two mirrors 74, 75, two half-wave plates 76, 77, anda Faraday rotator 78. Light from the amplifier 16 and the image relaytelescope 17 may be considered to contain two components: one linearlypolarized component derived from the input beam and not affected bybirefringence in the amplifier, and a smaller component that is linearlypolarized at an orthogonal angie, due to the birefringence effects. Thelight from the image relay telescope 17 impinges first on polarizer 72,which passes the first component, probably amounting to, e.g., 90% ormore of the light, and reflects the orthogonally polarized componentcaused by birefringence. For purposes of explanation, suppose that theinput light beam, i.e. the principal component transmitted by thepolarizer 72, is horizontally polarized and the birefringence componentreflected by the polarizer 72 is vertically polarized. The verticallypolarized light component is reflected by mirror 74, passes throughpassive half-wave plate 76 and Faraday rotator 78, and is "dumped" byreflection from the other polarizer 73. In the forward direction (to theright as illustrated), the combination of the half-wave plate 76 and theFaraday rotator 78 produce canceling rotations of the polarizationangle. Therefore, the birefringence component remains verticallypolarized and is reflected by the polarizer 73.

The principal or horizontally polarized beam has its polarization anglechanged to vertical by the lower half-wave plate 77, is next reflectedby mirror 75 and is reflected again by polarizer 73. Then the main beamis converted to circularly polarized light by the quarter-wave plate 18,and enters the SBS cell 20. Upon reflection from the SBS cell 20, themain beam is converted to horizontally polarized light by thequarter-wave plate 18 and is then transmitted by the polarizer 73. Onits return path, the principal beam component encounters the Faradayrotator 78 and the other half-wave plate 76, and in this direction thesetwo components combine to produce a 90° rotation in the polarizationangle. Therefore, the principal beam becomes vertically polarized, andis reflected from the mirror 74 and the polarizer 72.

Therefore, the beam returned to the amplifier 16 is orthogonallypolarized with respect to the input beam, and has had any birefringencecomponent removed. In the return pass through the amplifier 16, thepolarization will be rendered less "pure" again by the birefringenceeffect, but the configuration of FIG. 7c achieves a 50% reduction inbirefringence. If, for example, the birefringence component is as highas 10% for each amplifier pass, the invention reduces the overall effectof birefringence from 20% to 10%.

The master oscillator 22 may be of any suitable design that produces anear diffraction limited pulse output, but the preferred embodiment isshown in FIGS. 2a, 2b and 3. The master oscillator gain medium is ayttrium-aluminum-garnet (YAG) rod 80, which is side pumped bytwo-dimensional diode arrays 82, and has rod heat sinks 84 disposedabove and below the rod. The rod 80 has a highly reflective (HR) coating86 on its cylindrical surface opposite the diode arrays 82, and has anantireflective (AR) coating 88 on the opposite surface, adjacent to thediode arrays. There is also a parasitic suppression coating 90 tosuppress off-axis parasitics, such as those indicated at 92. Asindicated in the optical layout for the master oscillator (FIG. 3), themaster oscillator is disposed in a resonator cavity defined by a totalreflector 94 and an outcoupler 96. The resonator is injection seededfrom a seed laser 97, and is Q-switched by means of a polarizer 98 and aPockels cell 100, to provide output pulses at a desired rate. Theconfiguration outputs a near diffraction limited Q-switched pulse withexcellent temporal, amplitude and directional stability.

As shown in FIG. 8, beam shaping (12) is effected by the Faradayisolator 24 and one or more telescopes 102, 104. The Faraday isolator 24protects the master oscillator 22 from energy leaking through thepolarizer 14 due to birefringence in the amplifier 16. Thus, in theconfiguration of FIG. 7c, the Faraday oscillator 24 serves to removebirefringence components introduced during the return pass of the beamthrough the amplifier 16. The telescopes 102, 104 magnify the masteroscillator beam so that a uniform beam can be injected into theamplifier 16.

FIG. 9 shows how the invention can be implemented using well known imagerelaying principles and multiple amplifiers 16.1, 16.2 and 16.3. Spatialfilters 110.1 and 110.2 are disposed between adjacent amplifier stageshave oversized apertures (20 to 100 times the diffraction limit) andfunction to eliminate amplified spontaneous emission (ASE) and parasiticoscillations between amplifiers. Afocal image relaying telescopes,indicated by lenses 112, propagate the beam between stages and functionto eliminate the effects of diffraction from hard apertures such as theamplifiers. They also improve coupling efficiency between the amplifiersand maintain beam uniformity. The configuration of FIG. 9 illustrates amore practical embodiment of the invention but is not intended to belimiting.

FIG. 10 is a graph illustrating the performance of the apparatus of theinvention. In particular, the graph shows the variation of extractedenergy and beam quality with the master oscillator energy. The beamquality is relatively constant at 1.1 times the diffraction limit, overa wide range of master oscillator energy levels. Average powers of up to100 watts were recorded with this beam quality, providing an extremelybright beam of high quality.

It will be appreciated from the foregoing that the present inventionrepresents a significant advance in the field of high brightness lasers,and also in the field of zig-zag amplifiers. In particular, theinvention provides a laser architecture with extremely high brightnessand high beam quality, and a zig-zag amplifier without the drawbacks ofother amplifiers of the prior art. It will also be appreciated that,although a specific embodiment of the invention has been described indetail for purposes of illustration, various modifications may be madewithout departing from the spirit and scope of the invention.Accordingly, the invention should not be limited except as by theappended claims.

We claim:
 1. A high brightness solid-state laser source, comprising:amaster oscillator, generating a pulsed input beam having a nearlydiffraction limited beam quality; a zig-zag slab amplifier positioned toreceive and amplify the beam from the master oscillator, during a firstpass through the amplifier; an image relaying telescope positioneddownstream of the amplifier, as seen by the input beam; a phaseconjugation cell positioned to receive the amplified input beam from thezig-zag slab amplifier and to reflect the beam in phase conjugated formback into the zig-zag slab amplifier for a second pass, wherebyaberrations introduced in the zig-zag slab amplifier during the firstpass are practically canceled during the second pass; means forextracting an amplified beam after the second pass through theamplifier, whereby the extracted amplified beam has high beam qualityand extremely high brightness; a quarter-wave plate positioned adjacentto the phase conjugation cell, to convert linearly polarized light ofthe input beam to circularly polarized light for more effective use inthe phase conjugation cell, and to convert circularly polarized lightreflected from the phase conjugation cell into orthogonally polarizedlinear light for the return beam; and means for reducing the effect ofbirefringence introduced in the zig-zag amplifier; and wherein the meansfor extracting the amplified beam includes a polarizer to outcouple theorthogonally polarized return beam.
 2. A high brightness solid-statelaser source as defined in claim 1, wherein the means for reducing theeffect of birefringence includes:means for removing anybirefringence-caused component of the amplified input beam, transmittinga principal component of the amplified input beam to the quarter-waveplate, and transmitting a reflected beam back to the amplifier with apolarization angle orthogonal to that of the principal component of theinput beam.
 3. A high brightness solid-state laser source as defined inclaim 1, wherein the mans for reducing the effect of birefringenceincludes:first polarization-sensitive means located between the zig-zagamplifier and the quarter-wave plate, for transmitting a principalcomponent of the amplified input beam along a first optical path, andsimultaneously reflecting an orthogonal birefringence component of theamplified input beam along a second optical path; optical means locatedin the first optical path, for rotating the polarization angle of theprincipal component by 90°; means located in the second optical path,for transmitting light in the forward direction without change in thepolarization angle, and for rotating the polarization angle 90° forlight traversing the second optical path in the return direction,whereby the birefringence component of the input beam traverses thesecond optical path unaffected; and second polarization-sensitive means,located at a junction of the first and second optical paths, wherein theprincipal component of the input beam is reflected from the firstoptical path into the quarter-wave plate, the birefringence component isreflected into space and discarded, and the return beam from thequarter-wave plate is transmitted back along the second optical path;and wherein the return beam is subjected to a 90° rotation inpolarization angle by the optical means in the second optical path, andis reflected by the first polarization-sensitive means back toward theamplifier.
 4. A high brightness solid-state laser source as defined inclaim 3, wherein:the first and second polarization-sensitive means arepolarizers that reflect light that is linearly polarized in onedirection and transmit light that is linearly polarized in an orthogonaldirection.
 5. A high brightness solid-state laser source as defined inclaim 3, wherein:each of the first and second optical paths includes amirror.
 6. A high brightness solid-state laser source as defined inclaim 1, wherein the zig-zag amplifier further comprises:thermal controlbars positioned at upper and lower edge faces of the amplifier slab, toimprove temperature gradients in the vertical direction; andantireflective coatings on the upper and lower edge faces, to suppressparasitic light rays having a vertical component.
 7. A high brightnesssolid-state laser source as defined in claim 1, wherein the zig-zag slabamplifier includes:a slab of solid-state laser material, the slab havingtwo opposing sidewalls from which a light beam is repeatedly reflectedas it progresses longitudinally through the amplifier, and two opposingvertical end walls, oriented to receive the pulsed input beam at anangle approximately normal to the end walls, whereby input of light at anear normal angle of incidence minimizes polarization dependentreflection from the end walls; at least one diode array positioned inclose proximity to at least one of the sidewalls, to provide power tothe amplifier; and means for cooling the sidewalls by flowing a liquidin contact with the sidewalls.
 8. A high brightness solid-state lasersource as defined in claim 7, wherein:the means for cooling thesidewalls includes means for flowing the liquid in a longitudinaldirection past the sidewalls, to minimize temperature gradients in avertical direction.
 9. A high brightness solid-state laser source,comprising:a master oscillator, generating a pulsed input beam having anearly diffraction limited beam quality; a zig-zag slab amplifierpositioned to receive and amplify the beam from the master oscillator,during a first pass through the amplifier; an image relaying telescopepositioned downstream of the amplifier, as seen by the input beam: aphase conjugation cell positioned to receive the amplified input beamfrom the zig-zag slab amplifier and to reflect the beam in phaseconjugated form back into the zig-zag slab amplifier for a second pass,whereby aberrations introduced in the zig-zag slab amplifier during thefirst pass are practically canceled during the second pass; and meansfor extracting an amplified beam after the second pass through theamplifier, whereby the extracted amplified beam has high beam qualityand extremely high brightness;and wherein the zig-zag slab amplifierincludes a slab of solid-state laser material, the slab having twoopposing sidewalls from which a light beam is repeatedly reflected as itprogresses longitudinally through the amplifier, and two opposingvertical end walls, oriented to receive the pulsed input beam at anangle approximately normal to the end walls, whereby input of light at anear normal angle of incidence minimizes polarization dependentreflection from the end walls: at least one diode array positioned inclose proximity to at least one of the sidewalls, to provide power tothe amplifier; and means for cooling the sidewalls by flowing a liquidin contact with the sidewalls;and wherein the means for cooling thesidewalls includes means for flowing the liquid in a longitudinaldirection past the sidewalls, to minimize temperature gradients in avertical direction, and wherein the means for flowing the liquid in alongitudinal direction includes a transparent window positioned adjacentto and parallel to each sidewall, forming a channel for the flow ofcooling liquid; and a seal between the transparent window and thesidewall, the seal having transverse segments positioned in dead zonesof the sidewall on which no light falls; wherein the angle of incidenceof light internally on each sidewall is selected in relation to the beamwidth to provide dead zones on which no light falls.
 10. A highbrightness solid-state laser source as defined in claim 9, wherein:theslab further includes antireflective coatings on its end walls and onsidewall portions on which the input light beam first impinges, wherebyparasitic light rays approximately parallel with the longitudinaldirection are suppressed by the antireflective coatings.
 11. A highbrightness solid-state laser source, comprising:a master oscillator,generating a pulsed input beam having a nearly diffraction limited beamquality; an amplifier positioned to receive and amplify the beam fromthe master oscillator, during a first pass through the amplifier; aphase conjugation cell positioned to receive the amplified input beamfrom the amplifier and to reflect the beam in phase conjugated form backinto the amplifier for a second pass, whereby aberrations introduced inthe amplifier during the first pass are practically canceled during thesecond pass; means for eliminating birefringence effects introduced inthe first pass through the amplifier; and means for extracting anamplified beam after the second pass through the amplifier, including aquarter-wave plate positioned adjacent to the phase conjugation cell anda polarizer positioned between the master oscillator and the amplifier,whereby the quarter-wave plate ensures that circularly polarized lightis input to the phase conjugation cell and that the return beam isorthogonally polarized, whereby the extracted amplified beam has highbeam quality and extremely high brightness.
 12. A high brightnesssolid-state laser source as defined in claim 11, wherein the means foreliminating birefringence effects includes:means for removing anybirefringence-caused component of the amplified input beam, transmittinga principal component of the amplified input beam to the quarter-waveplate, and transmitting a reflected beam back to the amplifier with apolarization angle orthogonal to that of the principal component of theinput beam.
 13. A high brightness solid-state laser source as defined inclaim 11, wherein the means for eliminating the effect of birefringenceincludes:first polarization-sensitive means located between theamplifier and the quarter-wave plate, for transmitting a principalcomponent of the amplified input beam along a first optical path, andsimultaneously reflecting an orthogonal birefringence component of theamplified input beam along a second optical path; optical means locatedin the first optical path, for rotating the polarization angle of theprincipal component by 90°; means located in the second optical path,for transmitting light in the forward direction without change in thepolarization angle, and for rotating the polarization angle 90° forlight traversing the second optical path in the return direction,whereby the birefringence component of the input beam traverses thesecond optical path unaffected; and second polarization-sensitive means,located at a junction of the first and second optical paths, wherein theprincipal component of the input beam is reflected from the firstoptical path into the quarter-wave plate, the birefringence component isreflected into space and discarded, and the return beam from thequarter-wave plate is transmitted back along the second optical path;and wherein the return beam is subjected to a 90° rotation inpolarization angle by the optical means in the second optical path, andis reflected by the first polarization-sensitive means back toward theamplifier.
 14. A high brightness solid-state laser source as defined inclaim 13, wherein:the first and second polarization-sensitive means arepolarizers that reflect light that is linearly polarized in onedirection and transmit light that is linearly polarized in an orthogonaldirection.
 15. A high brightness solid-state laser source as defined inclaim 13, wherein:each of the first and second optical paths includes amirror.
 16. A high brightness solid-state laser source as defined inclaim 11, wherein the means for eliminating birefringence effectsincludes:a second polarizer, positioned to receive the amplified inputbeam from the amplifier and to separate the beam into a principalcomponent along a first optical path and a birefringence component alonga second optical path; a half-wave plate positioned in the first opticalpath, to rotate the polarization angle of the principal component by90°; a second half-wave plate and a Faraday rotator, positioned in thesecond optical path and having no net effect on the polarization angleof the birefringence component; and a third polarizer, positioned toreceive light from the first and second optical paths, wherein thebirefringence component from the second optical path is reflected intospace and discarded, and the principal component from the first opticalpath is reflected into the quarter-wave plate and is reflected as areturn beam by the phase conjugate cell back through the quarter-waveplate; wherein the third polarizer transmits the orthogonally polarizedreturn beam back along the second optical path, where the Faradayrotator and the half-wave plate effect a net 90° rotation of thepolarization angle, and wherein the second polarizer reflects the returnbeam back into the amplifier without any birefringence component.